This invention pertains to apparatus and methods for compensating distortions in membrane lithography masks, wafers, or both.
The size of circuit elements used in state-of-the-art integrated circuits continues to decrease. New lithography techniques will be needed to continue this reduction to sizes much smaller than those currently in use. Proximity X-ray lithography is a particularly promising technique, as it allows the largest exposure field of any of the contenders for the next generation of integrated circuit lithography, on the order of 5 cmxc3x975 cm in a single exposure. The large exposure field provides a significant throughput advantage, but it also makes image placement more critical, to the point where accurate image placement is widely regarded as a major factor limiting the use of X-ray lithography in very large scale integrated (xe2x80x9cVLSIxe2x80x9d) circuits. Overlay errors in proximity X-ray lithography may arise from several factors, including for example the following: (1) errors in the pattern writing tool; (2) distortions in the membrane mask caused, for example, by stresses in the absorber; and (3) distortions that are already present in the pattern on the wafer. Much effort has gone into minimizing all three effects, as well as at least partially compensating them by adjusting the magnification.
The industry""s response to this critical problem has typically been to design masks that are as rigid as possible, to try to minimize one potential cause of distortion at its source. While other types of masks are inherently more rigid, the inherent rigidity of membrane masks is relatively low. The rigidity of membrane masks has been increased, for example, by the use of diamond substrates. Membrane masks are currently required for X-ray lithography, ion beam lithography, and some types of e-beam lithographies. Although membrane masks may in principle be used in almost all lithography techniques, they have generally been considered less desirable. One approach in a projection electron beam system (the so-called SCALPEL system) has been to reinforce the required membrane masks with xe2x80x9cgrillagexe2x80x9d to increase their rigidity.
The type of xe2x80x9cmembrane maskxe2x80x9d used in X-ray lithography comprises a membrane and an absorber. The membrane is a continuous sheet that is relatively transparent to the radiation used to expose a resist on a wafer. The main function of the membrane is to support the absorber. The absorber, which adheres to the membrane, is relatively opaque to the radiation. The absorber is patterned to correspond with the pattern desired in the exposed and developed resist on the wafer, and need not be continuous since it adheres to the membrane.
In other lithographies, other types of membrane masks have been used. For example, the membrane may be opaque to the radiation, except where holes are placed in the membrane (so-called xe2x80x9cstencilxe2x80x9d masks). Alternatively, the absorber may be replaced by a patterned layer that scatters incident radiation instead of absorbing it.
Generally, the industry has addressed the problem of distortion by trying to manufacture masks that are as accurate as possible, considering the ideal to be features positioned on orthogonal, perfectly linear axes. Much of the cost of mask-patterning tools lies in the references, metrology, and feedback used to enhance accurate image placement. However, this approach cannot accommodate changes in a mask that occur in processing steps subsequent to resist exposure, nor accommodate changes that occur as a mask ages, nor match a mask to distortions that may exist on the wafer being exposed.
A technique called xe2x80x9cpattern-specific emulationxe2x80x9d has been used to compensate for distortion in X-ray masks, such as distortion caused in etching the absorber. In this method a xe2x80x9csend aheadxe2x80x9d mask is first made, and is then used to expose a level on a wafer. Pattern displacements from the desired positions are noted, and are fed back to the mask-writing tool. A new mask is then written incorporating these displacements. Although time consuming and costly, this method did improve overlay. See, e.g., A. Fisher et al., xe2x80x9cPattern transfer on mask membranes,xe2x80x9d J. Vac. Sci. Technol. B, vol. 16, pp. 3572-3576 (1998).
R. L. Engelstad and F. Cerrina of the University of Wisconsin have considered the displacement of features on a membrane mask by heated gas jets (private communication).
Magnification correction is considered one of the critical issues in lithography. In some lithographic techniques, magnification correction has been accomplished by an adjustment of the exposure tool. For example, in projection optical lithography it is routine to adjust the magnification by axial displacement of the reticle and refocusing in a projection system which is non-telecentric on the reticle side. Similarly, adjusting the gap in point-source X-ray lithography changes the magnification. Adjusting magnification is more difficult in storage-ring X-ray lithography.
It has also been proposed to correct for magnification errors by expanding or contracting either the mask or the wafer. Both mechanical and thermal means have been suggested for correcting magnification errors.
U.S. Pat. No. 5,155,749 discloses expansion of an X-ray membrane mask by heating a support ring to facilitate magnification matching between the mask and the wafer.
A method has been proposed to correct magnification errors by preheating the wafer, and then vacuum-chucking it so that its size is xe2x80x9cfrozen inxe2x80x9d by the chucking force when the wafer is cooled back down. See H. Aoyama et al., xe2x80x9cMagnification correction by changing wafer temperature in proximity X-ray lithography,xe2x80x9d J. Vac. Sci. TechnoL B, vol. 17, pp. 3411-3414 (1999).
U.S. Pat. No. 5,504,793 discloses a method for applying torque to an X-ray mask at several locations around its edge with mechanical actuators, to stretch or compress the mask membrane to provide magnification correction.
It has been proposed to mount the wafer on a spherical vacuum chuck, and to adjust the size of the front surface by changing the radius of the chuck via application of an internal pressure or vacuum. See M. Feldman et al., xe2x80x9cWafer chuck for magnification correction in X-ray lithography,xe2x80x9d J. Vac. Sci. Technol. B, vol. 16, pp. 3476-3479 (1998).
A novel method has been discovered, called the adaptive membrane mask technique, for locally heating the membrane in a lithographic mask to compensate for distortion in the mask, the wafer, or both. This technique may be used both to shrink and to expand areas of the mask, in order to adjust for varying magnitudes and signs of distortion. The adaptive membrane mask represents a major change in philosophy, from increasingly costly and difficult xe2x80x9cdead reckoningxe2x80x9d methods to one using feedback.
In one embodiment, the correction method comprises two steps: (1) A send-ahead wafer is exposed and measured by conventional means to determine the overlay errors at several points throughout the field. The errors may be the result of distortion in the mask, in the wafer, or both. (2) During exposure of subsequent wafers, calibrated beams of light are focused on the mask. The source of light may be, for example, a modulated laser beam, a halogen lamp, a capillary lamp, or a cathode ray tube. The light could have wavelengths from infrared to visible to ultraviolet. The light may be scanned across the mask, or projected on the mask directly or through a transparency or liquid crystal array, or produced by another projection system. The heating from the absorbed light produces displacements that compensate for the overlay errors measured with the send-ahead wafer. While heating a portion of a mask causes it to expand, a portion of a mask may also effectively be shrunk by heating the areas surrounding it.
In some circumstances, an alternative embodiment may be preferred. There is a delay inherent in the send-ahead wafer technique, due to the time required to develop the wafer, to measure its distortions, and to prepare a suitable transparency (if a transparency is used). The cost of the xe2x80x9cdown timexe2x80x9d for a lithography exposure tool, for example one on a synchrotron X-ray source, can be significant. In such a case, an alternative embodiment may be used to essentially eliminate such xe2x80x9cdown time,xe2x80x9d thereby reducing costs. Prior to using the exposure tool, distortions in the mask are measured off-line, and optionally the distortions in the wafer may be measured off-line as well. Based on the off-line measured distortions, the required compensations are calculated in advance, so that no time is lost while using the exposure tool itself
The novel compensation method allows any source of distortion to be correctedxe2x80x94for example, distortion appearing in the mask as manufactured, distortion that only develops in the mask over time, or distortion in the waferxe2x80x94in the latter case, particularly systematic distortions that are repeated from one wafer to the next.
This compensation method is simplified in the particular case of a scanned exposure using X-rays from an electron storage ring, since at any given time a correction need only be applied in the immediate vicinity of the line currently exposed by the X-ray beam. Two light beams may be used, one forming a line image just above, and one just below the X-ray beam. FIG. 1 illustrates one embodiment in accordance with the present invention in a storage ring beam line. The transparency partially transmits the light, and is scanned synchronously with the mask and wafer. Preferably, the lamp is a line source perpendicular to the plane of the paper. For clarity, only the beam forming a line image below the X-ray beam is shown; in practice some of the components could be shared by the light beams both above and below the X-ray beam. Intensity differences between the two beams cause differential mask expansion in the vertical direction, compensating vertical distortion, while intensity variations common to both beams compensate in the horizontal distortion; see FIGS. 7(a), 7(b), and 7(c): (It is understood that the designations xe2x80x9chorizontalxe2x80x9d and xe2x80x9cverticalxe2x80x9d refer to a direction parallel to the line being exposed by the radiation, and a direction perpendicular to that line, respectively.)
The loads produced by localized heating of the mask may be used to restore the mask to an undistorted condition, or to match an existing distortion pattern on the wafer. This ability gives membrane masks a flexibility that rigid masks lack. Instead of making the mask and the exposure tool as perfect and as fixed as possible, a task that is rapidly becoming a critical technical barrier to nanoscale and VLSI lithography, the adaptable mask in an image placement feedback system in principle allows the image placement accuracy to be made as good as the ability to measure it.
In another alternative embodiment, a mask may be fabricated and handled so that in use it is nearly free of distortion. This alternative embodiment preferably employs the following sequence of steps: (1) forming a fiducial grid on the mask prior to the formation of the masking pattern, preferably by interferometric lithography, and preferably in or on the membrane; (2) an optional step of characterizing the fiducial grid prior to formation of the masking pattern, for example by means of a holographic-phase-shifting interferometer, such as that shown for example in FIG. 8, and as otherwise described in M. Lim et al., J. Vac. Sci. Technol. B, vol. 17, pp. 2703-2706 (1999); and in K. Murooka et al., xe2x80x9cMembrane-mask distortion correction: analytical and experimental results,xe2x80x9d paper to be presented at the International Conference on Electron, Ion, and Photo Beam Technology and Nanofabrication (Palm Springs, Calif., May 30 to Jun. 2, 2000); (3) forming the masking pattern on the membrane, for example an X-ray absorber pattern in the case of an X-ray mask; (4) measuring the fiducial grid again, for example with a holographic-phase-shifting interferometer, to determine the distribution of distortion associated with, or caused by, the masking pattern; (5) calculating the stress distribution that produced the distortion distribution determined in step (4), and calculating from the stress distribution a heat-input distribution to compensate or correct the distortion distribution; and (6) applying the calculated heat input distribution to the mask.
Two techniques, an analytical method and a finite elements method, for calculating a heat-input distribution to compensate or correct a measured distortion distribution, are disclosed in (unpublished) pending proposal to DARPA BAA 00-4 (prepared Feb. 8, 2000), a complete copy of which is being submitted with this application as originally filed in the United States Patent and Trademark Office, and the complete disclosure of which is hereby incorporated by reference.
An advantage of using such a fiducial grid and the associated interferometric means of measuring distortion of the fiducial grid is that these measurements may be made while the mask is located in an exposure apparatus in preparation for exposure. Moreover, the measurement can be performed while the heat input is being applied, and hence during the actual exposure, allowing active feedback to the heat input.
FIG. 8 depicts a holographic phase-shifting interferometer that may be used in making these measurements. As compared to an interferometric lithography system such as is known in the art, the holographic phase-shifting interferometer has three primary modifications: (1) A fluorescent or non-fluorescent screen is placed in front of one pinhole to capture the interference pattern between the reflected and back-diffracted beams; (2) A piezoelectric transducer pushes the beam-splitter, which in turn causes a phase shift in one of the arms; and (3) A CCD camera (not shown) is used to record the fringe patterns.